ahuazhu commented on a change in pull request #1267: 采用java8 hashmap的散列算法
hashcode右移16位后与hashcode异或 散列效果更好
URL: https://github.com/apache/rocketmq/pull/1267#discussion_r294203051
##########
File path:
client/src/main/java/org/apache/rocketmq/client/producer/selector/SelectMessageQueueByHash.java
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@@ -25,12 +26,12 @@
@Override
public MessageQueue select(List<MessageQueue> mqs, Message msg, Object
arg) {
- int value = arg.hashCode();
- if (value < 0) {
- value = Math.abs(value);
- }
+ return mqs.get(indexForQueue(mqs.size(), arg));
+ }
- value = value % mqs.size();
- return mqs.get(value);
+ private int indexForQueue(int length, Object key) {
+ int h = (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
+ return h & (length - 1);
Review comment:
Maybe a bug :-)
When mqs.size() is not power of 2, some queue will never be selected.
for example. when mqs.size() is 5, only queue-0 and queue-4 could be
selected.
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